Paper mills transport a continuous web of paper through a complex array of rolls. Selected rolls in the paper mill are arranged in pairs and define a nip therebetween. Temperature, pressure, rotational speed and surface characteristics of the rolls determine the characteristics of the paper produced in the paper mill.
Many papers are required to have a smooth and/or glossy surface on at least one side. Hot soft nip calendering using a pair of specially covered rolls is commonly used in the prior art to impart a smooth or glossy finish to a surface of the paper being produced in the prior art paper mill. A prior art hot soft nip calender apparatus is identified generally by the numeral 100 in FIG. 1 and includes a pair of oppositely rotating rolls 102 and 104 defining a nip 106 therebetween. The roll 102 typically is formed from a metallic material, such as steel, and is a complex structure with passages 108 extending therethrough. A hot oil is circulated through the passages to heat the outer surface 110 of the roll 102. A temperature of 400.degree. F. often will be achieved in the nip 106. Other prior art rolls, however, are gas fired and may attain surface temperatures above 500.degree. F. The roll 104 in the prior art hot soft nip calender apparatus 100 includes a metallic shell 112 with a smooth soft outer cover 114 securely engaged or laminated thereon. The rolls 102 and 104 typically are disposed relative to one another to achieve an operating pressure on a paper web 116 therebetween in the range of 1,000-3,000 pounds per linear inch.
The high speed, high temperature and high pressure employed in the prior art hot soft nip calender apparatus 100 create a potential for failure of the cover 114 on the roll 104. In particular, the cover 114 is known to delaminate from the metallic shell 112 to which the cover 114 is initially affixed. A replacement roll often will cost in the range of $300,000- $400,000 and an additional expensive roll must be maintained in inventory. This high cost is due to the complicated lamination of the cover 114 to the shell 112 in an effort to achieve a soft surface that will withstand the high speeds, high pressures and high temperatures used in the prior art calender apparatus 100 described above. The delaminated cover 114 also can damage downstream equipment in the paper mill. Thus, the total cost for such a failure can exceed the significant cost of the soft calender roll 104 itself. Furthermore, the down-time for the paper mill can represent a substantial cost penalty independent of the replacement cost for the damaged roll.
Some aspects of paper finishing processes could be enhanced by using higher temperatures and/or pressure in the hot soft nip calender. For example, it often would be desireable to operate some such calenders at pressures approaching 2000 pounds per linear inch. It also would be desirable to achieve nip operating temperatures significantly higher than 400.degree. F. However, these higher pressures and temperatures would exacerbate the problems of delamination of the soft cover 114 from the shell 112 of the prior art apparatus 100.
In view of the above, it is an object of the subject invention to provide an improved hot soft nip calender.
It is another object of the subject invention to provide a hot soft nip calender where the soft smooth material of the nip can be substantially reduced in cost.
A further object of the subject invention is to provide a hot soft nip calender which enables higher operating temperatures and pressures in the nip.
An additional object of the subject invention is to provide an efficient hot soft nip calendering process.